A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device (or patterning structure), which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
One of the goals in integrated circuit fabrication is to faithfully reproduce the original design on the substrate (via the mask). As the demand to image smaller and smaller features in the semiconductor manufacturing process has continued unabated, the limitations of optical lithography that were once accepted have been exceeded repeatedly.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution R as shown in equation (a):
                    R        =                              k            1                    *                      λ            NA                                              (        a        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system and k1 is a process dependent adjustment factor.
It follows from equation (a) that the resolution can be improved in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1. All of these strategies have been pursued simultaneously in the past and are expected to continue to be pursued in the future.
The performance of a lithographic apparatus and its limitation may also be explained and characterized with the depth of focus (DOF), which is generally viewed as one of the most critical factors in determining the resolution of the lithographic projection apparatus. The DOF, defined in equation (b), is defined as the distance along the optical axis over which the image of the pattern is adequately sharp.
                    DOF        =                              +                          /                              -                                  k                  2                                                              *                      λ                          NA              2                                                          (        b        )            where k2 is an empirical constant.
Additional important responses/measures that provide more insight into the real difficulties associated with photolithography at the resolution limit include the exposure latitude (EL), the dense:isolated bias (DIB), and the mask error enhancement factor (MEEF). The exposure latitude describes the percentage dose range where the printed pattern's critical dimension (CD) is acceptable, typically 10%. It is used along with the DOF to determine the process window, i.e. the regions of focus and exposure that keep the final resist profile within prescribed specifications. As for the DIB, it is a measure of the size difference between similar features, depending on the pattern density. Finally, the MEEF describes how mask CD errors are transmitted into substrate CD errors.
As the semiconductor industry moves into the deep submicron regime, the resolution limit of currently available lithographic techniques is being reached due to a decrease in the depth of focus, difficulty in the design of projection system and complexities in the projection system fabrication technology. In order to address this issue, there have been continued endeavors to develop resolution enhancement techniques.
Historically, the resolution limit of a lithographic projection apparatus was optimized by the control of the relative size of the illumination system numerical aperture (NA). Control of this NA with respect to the projection system's NA allows for modification of spatial coherence at the mask plane, commonly referred to as partial coherence σ. This is accomplished, for example, through specification of the condenser lens pupil in a Köhler illumination system. Essentially, this allows for manipulation of the optical processing of diffraction information. Optimization of the partial coherence of a projection imaging system is conventionally accomplished using full circular illumination apertures. By controlling the distribution of diffraction information in the projection system with the illuminator pupil size, maximum image modulation can be obtained. Illumination systems can be further refined by considering variations to full circular illumination apertures. A system where illumination is obliquely incident on the mask at an angle so that the zero-th and first diffraction orders are distributed on alternative sides of the optical axis may allow for improvements. Such an approach is generally referred to as off-axis illumination.
Off-axis illumination improves resolution by illuminating the mask with radiation that is at an angle to the optical axis of the projection system. The angular incidence of the radiation on the mask, which acts as a diffraction grating, improves the contrast of the image by transmitting more of the diffracted orders via the projection system. Off-axis illumination techniques used with conventional masks produce resolution enhancement effects similar to resolution enhancement effects obtained with phase shifting masks.
Various other enhancement techniques that have been developed to increase the resolution and the DOF include optical proximity correction (OPC) of optical proximity errors (OPE), phase-shift masks (PSM), and sub-resolution assist features (SRAF). Each technique may be used alone, or in combination with other techniques to enhance the resolution of the lithographic projection apparatus.
One approach to generate off-axis illumination is to incorporate a metal aperture plate filter into a fly eye lens assembly of the projection system illuminator in order to provide oblique illumination. A pattern on such a metal plate could have two or four symmetrically arranged openings (zones) with sizing and spacing set to allow diffraction order overlap for specific geometry sizing and duty ratio on the mask. Such an approach results in a significant loss in intensity available to the mask, lowering throughput and making the approach less than desirable. Additionally, the two or four circular openings need to be designed specifically for certain mask geometry and pitch and do not improve the performance of other geometry sizes and spacings. See, for example, EP 0 500 393, U.S. Pat. Nos. 5,305,054, 5,673,103, 5,638,211, EP0 496 891 and EP0 486 316.
Another approach to off-axis illumination using the four-zone configuration, which is disclosed in U.S. Pat. No. 6,452,662, is to divide the illumination field of the projection system into beams that can be shaped to distribute off-axis illumination to the mask. By incorporating the ability to shape off-axis illumination, throughput and flexibility of the lithographic apparatus may be maintained. Additionally, this approach allows for illumination that combines off-axis and on-axis (conventional) characteristics. By doing so, the improvement to dense features that are targeted with off-axis illumination is less significant than straight off-axis illumination. The performance of less dense features, however, is more optimal because of the more preferred on-axis illumination for these features. The result is a reduction in the optical proximity effect between dense and isolated features. Optimization is less dependent on feature geometry and more universal illumination conditions can be selected.
Referring to FIGS. 3a–d, currently available illumination intensity distributions or arrangements include small, or low, sigma (FIG. 3a), annular (FIG. 3b), and off-axis quadrupoles (FIGS. 3c–d), with the illuminated areas (hereinafter referred to as the aperture(s)) shown in cross section. In FIG. 3d, the off-axis quadrupole illumination includes four substantially identical poles arranged at +/−45° relative to the horizontal axis of the illuminator. Each of these poles corresponds to a segment of an annular illumination and may be obtained with the optical assembly of FIG. 10. This illumination may be referred to as “QUASAR” type illumination in the remaining text. The annular, quadrupole and QUASAR illumination techniques of FIGS. 3b–d are examples of off-axis illumination schemes.
Small sigma illumination is incident on the mask with approximately zero illumination angle (i.e. almost perpendicular to the mask) and produces good results with phase shifting masks to improve resolution and increase the depth of focus. Annular illumination is incident on the mask at angles that are circularly symmetrical and improves resolution and increases depth of focus while being less pattern dependent than other illumination schemes. Quadrupole and QUASAR illumination are incident on the mask with four main angles and provide improved resolution and increased depth of focus while being strongly pattern dependent.
Referring to FIGS. 4 and 5, two illumination systems are schematically illustrated. The systems illustrated in FIGS. 4 and 5 include light collecting/collimating optics 10; an axicon/zoom module 12; and light integrating and projecting optics 14. The illumination systems include an optical axis 16, a pupil plane 31, and a mask plane 20. The axicon/zoom module 12 comprises a pair of axicons 33, one concave and one convex, whose separation can be varied. The module 12 also comprises a zoom lens 24.
For the case of conical axicons, some examples of the illumination intensity distributions achievable at the pupil plane 31 are shown in FIG. 6. The spot size can be varied between states A and B by changing the zoom lens position. Similarly, the annularity can be changed between states A and C by varying the axicon opening (separation between the axicons).
To improve the illumination homogeneity, an optical integrator 26 is used. In FIG. 4 the optical integrator takes the form of a light pipe 26, such as a glass, calcium fluoride or quartz rod. A coupler 28 couples the illumination at the pupil plane 31 into the rod 26, and rod exit imaging optics 30 are also provided. In FIG. 5 a fly's eye element 32 acts as the integrator. The fly's eye element 32 is a composite lens comprising an array or honeycomb of small lenses. Further, objective lenses 34 and 36 complete the projection optics 14.
However, the creation of complex source shapes to optimize the optical transfer of a given mask pattern remains a slow and expensive process. Even the production of small sigma poles, e.g. smaller than 0.1, with commercially available beam shapers can potentially be complicated. Complex source shapes that cannot be generated with, for example, an axicon module generally require the use of custom elements, which are specifically designed to provide a particular intensity distribution at the pupil plane of the illuminator. Unfortunately, these custom elements are expensive and cannot generally be modified if they do not provide the intended results.